Dry process for making composite consolidated products with controlled presteaming of the raw materials



1952 E. G. HALLONQUIST 8 ,654

DRY PROCESS FOR MAKING COMPOSITE CONSOLIDATED PRODUCTS WITH CONTROLLED PRESTEAMING OF THE RAW MATERIALS Filed NOV. 21, 1949 INVENTOR. $3790 709 alzv/u Ear/EJ9411 latented Jan. 8, 1952 DRY PROCESS FOR MAKING COMPOSITE CONSOLIDATED PRODUCTS" WITH CON TROLLED PRESTEAMING OF THE RAW MATERIALS I Earl G. Hallonquist, Tacoma, Wash, assignor to Plywood Research Foundation, Tacoma, Wasln, a nonprofit corporation Application November 21, 1949, Serial No. 128,664

3 Claims. (01. 18-475) This invention relates to a process for making composition boards and related products by the consolidation of dry felts or mats composed of pieces, particles or fibers of ligno-cellulose materials, particularly wood fibers.

Processes for the production of consolidated composition boards, particularly hardboard, may

be of two general classes. In the wet process, a

low consistency slurry of wood fiber in water containing roughly from 3,000 to 10,000% by weight water (on the basis of the dry fiber) is passed through a screen to form a felt from the suspended fibers. This felt then is consolidated to the desired degree by the application of heat and pressure.

In a dry process, on the other hand, the solid fibers are formed into a felt by mechanical means without suspension in a liquid medium. The fibers are not dry, however, in the literal sense of not containing any moisture. They may contain substantial amounts of moisture, such as,

for example, in the order of 5 to 150%, such moisture content obtaining because of the moisture content of the wood and of the added materials. It is with a dry process that the present invention is concerned, the words dry process and dry forming into a mat being used herein in the accepted sense described above, i. e., in the sense that the li-gnocellulose raw material is handled in the form of solid pieces or particles without suspending it in a liquid vehicle, although the lignocellulose mixture, just prior to pressing, may have a substantial moisture content. A suitable dry process in which the process of the present invention is applicable, is described in the co-pending application of Schubert et al., Serial No. 51,938, filed September 30, 1948.

In both of the foregoing procedures, the wood or other cellulose material often is subjected to a treatment with steam under pressure with or without added chemicals in order to facilitate its reduction into fibrous form and to produce soft fibers of improved quality and board making properties. Because of the sensitivity of lignocellulose materials to the action of high pressure steam, a certain amount thereof is made water soluble by such a treatment, the products of the reaction being principally water soluble polysaccharides. Such products are familiarly referred to by the general term water solubles, this term being used herein in this accepted sense.

If the mixture of fibers and water solubles produced by a defibering operation preceded by or performed contemporaneously with a steaming operation is employed in the manufacture of conmation of the water solubles has little influence on the process per se as such water solubles are completely separated and washed out from the fiber by the very large volumes of Water used as the suspending medium. However, this has the disadvantage of causing the loss of a substantial proportion, i. e., from about 5 to 20%, of the lig'nocellulose raw material and thus reduces yield.

The situation is otherwise when the mixture of fibers and water solubles is formed into a mat or felt by dry felting procedures and then consolidated in a hot press. In this operation any water solubles which are present are retained in the fiber and subjected to the elevated conditions of temperature and pressure prevailing in the press. 'I'hereupon they are entrained in part by the squeeze out water deriving from the moisture content of the lignocellulose mixture and, upon evaporation of such moisture, are deposited in the screen and on the platens of the press. 'In addition to clogging the screen and making it stick to the platen, they block the passage of steam formed within the felt. Even more important, however, they are subject to flow under the pressing conditions so that the quality of the consolidated product is degraded by the formation of flow areas and spots on its surface.

It therefore will be apparent that, in a dry process of making consolidated composite products, it is of the utmost importance that the water solubles content of the felt prior to consolidation be maintained at a minimum. This necessity is inconsistent, however, with the desirability of presteaming the lignocellulose raw material prior to defibration in order to expedite the latter process and to improve the quality of the fiber. As explained above, the presteaming operation necessarily results in the conversion of a substantial proportion of the lignocellulose material into water soluble products.

It has been discovered as a solution to the above problem that a critical relationship exists between the pressure at which the presteaming operation is carried out and the formation of water soluble products by such an operation. At least within the ordinary operating schedules, this effect seems to be substantially independent of steaming duration, the steaming pressure being the factor of primary significance in determining the amount of water soluble products formed.

' Hence it appears that there is within lignocelsolidated products by the wet process, the forlulose materials a content which is broken down rapidly by the hydrolytic action of steam above a certain pressure level.

As is shown in the accompanying drawing,

wherein the amounts of water solubles are plotted against the steam pressure, the amount of such materials formed increases very slowly at steam pressures of from about atmospheric pressure up to about 75 p. s. i. (the steam pressures stated herein are gauge pressures of saturated steam). Hence the presteaming operation may be carried out desirably anywhere within this pressure range. Pressures of below about 20 p. s. i. are not preferred since the desired degree of softening of the lignocellulose and fiber improvement may not be obtained, and pressures of above '15 .p. s. i. are undesirable because of the ver pronounced increase in water solubles production. A preferred pressure range for eifectuating the steaming operation for most lignocellulose materials lies be tween the limits of about p. s. i. to about '60 .lulose material (wood) was reduced to the form -of'chips and subjected to the action of steam at the indicated pressures and corresponding temperatures for saturatedsteam in a pressure chamber .of conventional construction. The steamed .chi-ps then were defibered'in a mechanical grinder. The water solubles content of the fiber produced from the steamed chips was determined by 'boiling a 12 to '15 ,gram sample (oven dry Weight) for 10 minutes in 600 ml. of water. The boiled fiber then was filtered, washed with 300 ml. of hot water and dried at '105-C. to constant weight. The loss in weight due to the extraction with boiling water-then was calculated as being the water solubles-content of "the sample.

The results .of .a series of determinations at a constant steaming period Of thirty minutes but at varying steam pressures are -.givenin Table I.

Table I The results of a series of experiments carried out :to determine the effect of steaming :time on water solublesproduction is given-in Table II, the steam pressure being held constantat 100 .p..s. i. in each of these determinations.

Table II Water'Sol- Steaming ubles Example No. Time, Produced,

Min. Percent by weight It is apparent from an examination of-thedate presented in the above tables that, when the steaming pressure is kept below about '75 p. s. i., the formation of water solubles is kept at a minithis effe t being substant a l indep t of the. time of steaming within-the time limits set forth.

Therefore, in the production of consolidated fibrous products by the dry process of the present invention, raw lignocellulose material including annual products such as straw, cane, and cornstalks as well as the wood of various species of trees first is "reduced to a convenient particle size such as chips or hog fuel. The chips are introduced into a pressure chamber which may ;.or :may not be provided with agitating means.

They then are steamed at pressures of between about atmosphericand about '75 p. s. i., preferably between about 20 p. s. i. and about 60 p. s. i., and corresponding temperatures for saturated steam for a period oftime sufficient to accomplish thedesiredsoftening of the chips and improvement of the fibers. The time required for this is somewhat variable depending upon the particular raw lignocellulose material employed, but in the case of wood, for example, it will be from about 10 minutes to about 30 minutes.

After the steaming is completedrfihe steamed chips are transferred to an Allis-Chalmers or Bauer grinder or other defiberer of suitable construction. Additive materials including thermosetting binders, wax emulsions (for sizing), fire proofing materials, fungicides and the ,likeQmay be included atthispoint if desired. In this manner, the grinding operation is made to serve, the double function of reducing the chips to fibers while contemporaneously thoroughly intermixing the additives therewith. The additive materials may lee-added subsequently to the grinding, however, by any appropriate mixing method if this is desirable inagiven case. j

The fiber produced by the grinding operation, together with the additive materials, then is formed into a felt of uniform dimensions by any suitable .dry felting mechanism. The resulting felt :may be precompressed, if desired, and intro-' iiuced dry .into a 'multi-p'laten press of conventional construction.

In :the press, consolidating pressures .and temperatures are employed suitable .for the particular binder and lignocellulose :material of .which the-felt is comprised andfor-the particular density desired in the final consolidated Product. When producing :a one-quarter-inch hardboard having a density of about .1, using .wood fiber and la -phenol-formaldehyde thermosetting resin as .a binder, pressures ofrfrom to 800 p. sni. (preferably about 200 .p. s. i.) and temperatures of :from 1toZ00 C. "(preferably about C.)

steam 'through'the felt to the exterior of the press. in addition, theconsolidated products are free from flow areasor blemishes causedby the now of water solubles in the felt. Still further, the practice of the present invention results in maximum utilization of the lignocellulose raw material since a minimum amount of'the latter is converted to water solubles of which only a small proportion is lost with squeeze out water, if any resulting during pressing. The invention thus presents a distinct advantage over conventional wet processes in which relatively very large quantities of water solubles are formed and almost completely lost in the wash water and the squeeze out water. Yields in the order of 95% are not uncommon with the present invention, while yields in conventional wet processes are often in the order of 75%. Thus this invention results in very high yields of finished products from a given amount of lignocellulose starting material.

Having now described the invention in preferred embodiment, what is claimed is:

1. The process for the production of consolidated lignocellulose products which comprises directly exposing raw lignocellulose to steam at pressures of from about atmospheric to about 75 p. s. i. and from about ten to about thirty minutes thus only to soften the fibers and render the lignocellulose more susceptible to defibering while maintaining at a minimum the formation of water soluble products from the lignocellulose, reducing the steamed lignocellulose to the form of small pieces, dry forming the resulting pieces into a mat, and consolidating the mat by the application of consolidating temperatures and pressures.

2. The process for the production of consolidated lignocellulose products which comprises directly exposing raw lignocellulose to steam at pressures of from about 20 p. s. i. to about 60 p. s. i. and from about ten to about thirty minutes thus only to soften the fibers and render the lignocellulose more susceptible to defibering while maintaining at a minimum the formation of water soluble products from the lignocellulose, reducing the steamed lignocellulose to the form of small pieces, dry forming the resulting pieces into a mat, and consolidating the mat by the application of consolidating temperatures and pressures.

3. The process for the production of consolidated lignocellulose products which comprises directly exposing raw lignocellulose to steam at pressures of from about atmospheric to about p. s. i. and from about ten to about thirty minutes thus only to soften the cellulosie fibers and render the lignocellulose more susceptible to defibering without forming water soluble products by the hydrolysis from the lignocellulose, defibering the steamed lignocellulose, forming a mixture of the resulting fibers and a thermosetting binder, dry forming the said mixture into a mat, and consolidating the mat to a product of substantial density by subjecting it to pressures of from about to about 800 p. s. i. and temperatures of from about C. to about 200 C. for times of from about 10 minutes to about 30 minutes.

EARL G. HALLONQUIST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Chemical Abstracts, vol. 37 (1943), page 3933, abstract of article by Schrader. 

1. THE PROCESS FOR THE PRODUCTION OF CONSOLIDATED LIGNOCELLULOSE PRODUCTS WHICH COMPRISES DIRECTLY EXPOSING RAW LIGNOCELLULOSE TO STEAM AT PRESSURE OF FROM ABOUT ATMOSPHERIC TO ABOUT 75 P.S.I. AND FROM ABOUT TEN TO ABOUT THIRTY MINUTES THUS ONLY TO SOFTEN THE FIBERS AND RENDER THE LIGNOLCELLULOSE MORE SUSCEPTIBLE TO DEFIBERING WHILE MAINTAINING AT A MINIMUM THE FORMATION OF WATER SOLUBLE PRODUCTS FROM THE LIGNOCELLULOSE, REDUCING THE STEAMED LIGNOCELLULOSE TO THE FORM OF SMALL PIECES, DRY FORMING THE RESULTING PIECES INTO A MAT, AND CONSOLIDATION THE MAT BY THE APPLICATION OF CONSOLIDATING TEMPERATURE AND PRESSURES. 